1. Field of the Invention
This invention relates to the fabrication of millimeter and submillimeter wavelength devices, and more particularly the fabrication of millimeter and sub-millimeter wavelength horn antennas integrated with waveguides, channels, and other components using lithographic and etching techniques.
2. Discussion of Background
In general terms, an electromagnetic waveguide is any structure which is capable of confining and guiding electromagnetic energy from one point to another in a circuit. A variety of structures have been devised to accomplish this goal. For example, coplanar waveguide is a type of waveguide which consists of thin strips of coplanar conductive material on a dielectric substrate. Another example is dielectric waveguide in which the radiation is confined in a coaxial dielectric tube by the principle of total internal reflection. A hollow metal electromagnetic waveguide is an electrically conductive hollow tube or pipe-like structure or a collection of such structures designed to confine and guide electromagnetic radiation. A horn is a tapered or flared waveguide structure which couples energy to or from free space and concentrates the energy within a defined spatial distribution (beam pattern). Only the inside surface of these structures must be conductive as the major fraction of the electrical current is constrained by nature to flow within a thickness known as the skin depth which is directly related to wavelength. Also the inner dimensions of such waveguides are determined by the radiation wavelength and are also generally proportional to wavelength.
Because of these relationships, the fabrication and design of hollow waveguides is strongly dependent on the operating wavelength. For example, in the case of microwaves with wavelengths on the order of centimeters, hollow waveguides can be easily fabricated by the extrusion of rectangular metallic tubes which have inside dimensions on the order of centimeters. Injection molded or extruded plastic waveguide components are also typically easily made for microwave wavelengths if they are coated with a sufficiently thick conductive material on internal surfaces. Also waveguide components for microwave frequencies can be made in sections which are joined by flanges and alignment is typically not difficult because of the relatively large dimensions.
However, the fabrication of hollow waveguide assemblies for millimeter and submillimeter wavelengths is typically much more difficult because the dimensions are correspondingly smaller. Also assemblies and subassemblies of waveguides must be combined with active electronic devices such as diodes or transistors and other passive components and circuits to make radio receiver and transmitter components such as heterodyne mixers. Therefore a complex network of accurately aligned, interconnected and very small hollow metal channels must be made and some of these channels must hold active and passive electronic components. This is generally not feasible with microwave style tubing.
A waveguide assembly designed for millimeter and submillimeter wavelengths is traditionally made by fabricating two machined metal "half" blocks, which when joined together, to form a structure comprised of air-filled metal channels. Because of RF electromagnetic field and current considerations, it is rare that any of the slots can typically be formed only in one half with the other half being a simple flat cover. Thus the blocks have slots of various shapes and sizes which are often the mirror image of each other and which require precise control of depth, width and position (i.e., alignment). This "split block" approach solves two basic problems: (1) the difficulty of monolithically forming complex and very small hollow metallic structures and (2) the need to insert a circuit deep within the structure.
In recent years high quality millimeter and submillimeter wavelength components have been manufactured using a technique based on direct machining of metal blocks, for example, as described by Siegel et al., "Measurements on a 215 GHz Subharmonically Pumped Waveguide Mixer Using Planar Back-to-Back Air-Bridge Schottky Diodes", IEEE Trans. Microwave Theory and Tech., Vol. MTT-41, No. 11, pp. 1913-1921, November 1993, and Blundell et al., "Submillimeter Receivers for Radio Astronomy", Proc. IEEE, Vol. 80, No. 11, pp. 1702-1720, November 1992. FIG. 7 of Blundell et al is a drawing of machined horn antenna and waveguide fabricated using the described technique. A horn antenna is commonly used to couple electromagnetic radiation into the waveguide in communications applications. The primary benefits of machining the waveguide and the horn antenna into the metal block are that it is a well understood process which gives the designer great flexibility, the final structure is robust, and all internal components, such as semiconductor diodes, are protected from the environment. In addition, the machining process is essentially three dimensional, and therefore allows the integration of electromagnetic horns of nearly arbitrary shape.
Although the above-described direct machining technique has gained wide industry acceptance, the expense of the required machining equipment, the personnel expertise, and the fabrication time greatly increase the cost of fabricating millimeter and submillimeter wavelength components. Also, as the desired operating frequency of the components is increased (i.e., wavelength is decreased), the required dimensions of the metal block features shrink proportionally in relation to the decrease in wavelength, making fabrication even more costly and difficult.
Another common technique for fabricating millimeter and submillimeter wavelength components is known as electroforming, for example, as described by Ellison et al., "Corrugated Feedhorns at Terahertz Frequencies-Preliminary Results", Fifth Intl. Space THz Tech. Symp., Ann Arbor, Mich., pp. 851-860, May 1994. In the electroforming technique, a metal mandrel is formed by high precision machining techniques and is then used as a metal core around which a second metal is deposited by electroplating. It is this second metal which eventually forms the hollow metal waveguide after the initial metal is chemically etched away. This technique is employed because it is often easier to machine the mandrel than the actual waveguide itself. Using this technique, components have been fabricated for frequencies up to 2.5 THz, however, the fabrication of the components is still costly and difficult.
Another technique for fabricating millimeter and submillimeter wavelength horn antennas is known as silicon micromachining, for example, as describe by Ali-Ahmad, "92 GHz Dual-Polarized Integrated Horn Antennas", IEEE Trans. Antennas and Prop., Vol. 39, pp. 820-825, July 1991, and Eleftheriades et al., "A 20 dB Quasi-Integrated Horn Antenna", IEEE Microwave and Guided Wave Letters, Vol. 2, pp. 73-75, February 1992, which are incorporated herein by reference. Using this technique, and as in the present invention, the horn antennas are fabricated using a preferential/selective wet etch and silicon wafers with a correct crystal orientation, such that the etch process proceeds very quickly in the vertical or (100) crystal plane direction but which virtually stops when the (111) crystal planes are. When the etch is carried to completion, only the (111) plane surfaces are exposed, and the result is a pyramidal shape etched into the silicon having a flare angle between two opposite sides of the pyramidal shape of about 70 degrees. Although the pyramidal shape etched into the silicon can be used to fabricate a horn antenna, the wide flare angle of 70 degrees causes the horn antenna to have an unacceptably poor directivity (i.e., the beam is very broad). To compensate for this problem, Eleftheriades et al teaches attaching external metal sections having much smaller flare angles to the micromachined horn antenna to increase directivity. However, since these additional sections need to be machined and aligned to the pyramidal shaped horns, much of the benefit of silicon micromachining is lost.
Using quasi-optical techniques, for example, as described by Rebeiz, "Millimeter-Wave and Terahertz Integrated Circuit Antennas", Proc. IEEE, Vol. 80, No. 11, pp. 1748-1770, November 1992, the need for waveguides and horn antennas is completely eliminated. Instead, a traditional antenna is used to couple free-space electromagnetic radiation directly to the microelectronic device in use. This techniques has not yet given as good results as is possible with machined waveguides and horns, and is not yet accepted by the millimeter and submillimeter wavelength community, usually because of a lack of mechanical robustness in devices fabricated using this technique, susceptibility to electromagnetic interference, and the relatively large size of quasi-optical components.
Another technique for fabricating communication components is, for example, monolithic microwave integrated circuit (MMIC) technology, for example, as described by Bahl, "Monolithic Microwave Integrated Circuit Based on GaAs MESFET Technology", in Compound Semiconductor Electronics, The Age of Maturity, Ed. M. Shur, World Scientific, pp. 175-208, 1996. MMIC technology uses fully planar processing to form circuitry on wafers with planar waveguides, such as microstrip or coplanar waveguide, rather than hollow metal waveguides. Although this technology is very useful for fabricating devices operating at microwave frequencies (i.e., typically less than 30 GHz), MMIC technology has not yet been useful for fabricating devices operating at frequencies above about 100 GHz. This technique suffers from high losses due to the properties of the substrate materials and the poor characteristics of planar antennas manufactured using this technique as compared to horn antennas manufactured using other techniques.
Techniques using photoresist formers to fabricate waveguides and horns, for example, as described by Treen et al, "Terahertz Metal Pipe Waveguides", Proc. 18th Intl. Conf. on IR and Millimeter Waves, pp. 470-471, September 1993, Brown et al, "Micromachining of Terahertz Waveguide Components with Integrated Active Devices", Proc. 19th Intl. Conf. on IR and Millimeter Waves, pp. 359-360, October 1994, and Lucyszyn et al, "0.1 THz Rectangular Waveguides on GaAs Semi-Insulating Substrate", Electronic Letters, Vol. 31, No. 9, pp. 721-722, April 1995. Techniques using photoresist formers to fabricate waveguides and horns take advantage of techniques developed by the silicon microelectronics industry. Using this technique, hollow metal waveguides and horns formed around appropriately shaped layers of photoresist have been fabricated. The benefit of this technique is that the processing and shaping of photoresist is a well developed technology which can be precisely controlled on large wafers, thereby allowing many structures to be manufactured simultaneously and thus reducing costs. Also, photolithographic techniques easily allow the precision necessary for waveguide structures at the highest frequencies envisioned. The primary problems with photoresist technology have been forming and processing tall enough photoresist structures cheaply and reliably, removing the thick photoresist from inside the waveguides, because most of the surface area of the resist is not exposed to the solvent but rather covered by the waveguide, and only horns that flare in one dimension are possible, because the horns are flat, resulting in waveguides and horns having poor beam quality.
A new class of photoresist, EPON SU-8, for example, as described by Lee et al., "Micromachining Applications of a High Resolution Ultrathick Photoresist", J. Vac. Sci. Technol. B13(6), pp. 3012-3016, November/December 1995, appears to have solved the first problem of forming and processing tall enough photoresist structures cheaply and reliably. A preferential etching technique, for example, as described in U.S. Provisional Application No. 60/041,669 filed Mar. 25, 1997, by Koh et al entitled "A Preferential Crystal Etching Technique for the Fabrication of Millimeter and Submillimeter Wavelength Horn Antennas", offers a solution to the remaining problems. Using this technique, a cavity is preferentially etched in a substrate through a mask opening and the horn length and flare angle .theta..sub.1 are determined by a shape of the mask opening which is controlled by a photolithography process, and the etch depth is determined by the mask shape, rate of the etch, and the etch time.
The present invention takes advantage of the development of new photoresist materials which easily form features of the appropriate size and complexity, for example, as described by Lee et al above, and the development of micromachining techniques based on crystallographic etches which can form three dimension etched structures, for example, as described by Koh et al above, both of which are incorporated herein by reference, to allow fabrication of millimeter and sub-millimeter wavelength horn antennas integrated with waveguides, channels, and other components.